1. Field of the Invention
The present invention relates to a concentrated winding coil for use in an electric motor for example. Furthermore, this invention relates to a method of manufacturing the concentrated winding coils.
2. Description of Related Art
In a stator, comprising a core and a winding wire, used for various kinds of electric motors, winding density of the wire has been required to increase in order to pursue high efficiency of the electric motor.
FIG. 1 is a schematic illustration showing top and side views of a conventional concentrated winding coil disposed in a core. FIG. 1 shows a positional relationship between a coiled bobbin 10 and a core 5, wherein a wire 2 is wound around a bobbin 1 in the core 5. As shown in the top view, in order to dispose the coiled bobbin 10 closely adjacent to the core 5 that is arranged circular in the final assembly, the circumferential length of the bobbin's outer flange 1-b is longer than that of the bobbin's inner flange 1-c. Also, as shown in the side view, the coiled bobbin 10 includes a coil slot section 10a located inside the core 5 and a coil end section 10b located outside the core 5. Since the coil slot section 10a located inside the core 5 greatly influences characteristics of an electric motor, the coil slot is a rectangle shape having a long side on the coil slot section 10a side and a short side on the coil end section 10b side as shown in the cross sectional view of FIG. 2.
FIG. 2 is a schematic illustration showing a top view and a cross sectional view cutting along A-A line in the top view of an example of a conventional bobbin 1 used for a concentrated winding coil. The outer flange 1-b and the inner flange 1-c are configured respectively on each side of the bobbin body 1-a, around which a wire 2 is wound, so as to prevent the wire 2 from removing from the bobbin body 1-a. As shown in the cross sectional view, an R (round) portion is provided on each of the four corners of the bobbin body 1-a. The radius of the R portion is usually determined according to the flexibility of the wire 2 and the strength of coating layer (insulator) of the wire 2.
By referring to FIGS. 3 through 6, problems will be explained, which occur when a wire is wound around the above-mentioned bobbin 1. FIG. 3 is a model drawing for explaining the winding problem. Herein, side A and side C are of the coil slot section, side B and side D are of the coil end section, and a terminal wire is pulled out in the direction of side D. For purposes of simplifying, each side is drawn with the same length. The winding method is: a wire is wound around the bobbin by the rotation of the bobbin with regard to a wire nozzle (not shown).
FIG. 4 is a drawing for explaining a relationship between a wire position in a coil layer and a side of the bobbin. FIG. 4 shows the trajectory of the wire in the n-th turn and the (n+1)-th turn (“n” is a natural number) on each side of the bobbin when guiding of the wire 2 in the coil axis direction (expressed as “wire traverse”) with a constant rate is provided in synchronization with a rotation angle of the bobbin. Basically, in alignment winding, the wire 2 of the (n+1)-th turn is wound in close contact with the previous turn (n-th turn) not to have a gap therebetween in order to increase the winding density. In this case, however, useless space inevitably generates at a start and an end portions of the coil layer since the wire is wound in a regular helical form. Specifically, there are problems in that the slot-fill rate (ratio of the total wiring area to the winding space) decreases in the coil slot section on side A and side C, thereby affecting (degrading) the characteristics of the electric motor.
Accordingly, in order to solve the above problems, a method of guiding the wire 2 in the coil layer has been presented. FIG. 5 is a drawing for explaining another relationship between a wire position in a coil layer and a side of the bobbin. As shown in FIG. 5, this is a method in which wire traverse is not executed between side A and side C but is executed only on side D, thereby increasing the slot-fill rate in the coil slot section on side A and side C.
FIG. 6 is an explanatory drawing that shows an actual winding state with occurrence of a wire drifting. As shown by the broken line in the drawing, when wire traverse from side A through side C is set at 0 and wire traverse of the same quantity as a winding pitch is applied only on side D, an actual trajectory of the wire is prone to be as shown by the solid line. This is because the wire 2 is wound around the bobbin 1 while the wire is being pulled, and as shown in a cross sectional view of FIG. 2, this is also because the wire 2 tends to slip due to the R portion located on four corners of the bobbin body 1-a. Then, a component force of the tension is applied to the wire 2 so that the shortest distance is taken. Consequently, disturbance in wire alignment (e.g., a wire drifting) occurs only on side C, or in the range from side C to side B, or from side C to side A.
To solve the above problems, as described in JP-A-2003-244906, a method has been proposed in which a pair of wire guides called “former” are disposed on the outer circumference of the winding bobbin, and a wire is put through the guides and is wound around the winding bobbin. In this method, the former is approximated to the bobbin's flanges and is moved from the inner flange to the outer flange with winding the wire, thereby increasing the accuracy of positioning the wire on the winding bobbin.
FIG. 7 is a schematic illustration showing cross sectional views at a slot portion of another example of a conventional bobbin and a concentrated winding coil wound on the bobbin in which a process of wire winding until the third coil layer is illustrated. As shown in FIG. 7, a bobbin 1 has grooves that correspond to the wire pitch on the bobbin body 1-a. By applying a wire guide method, shown in FIG. 5, when a wire 2 is wound around the bobbin along the grooves, it is possible to ensure a good alignment on the side that is incorporated into the core slot. This is made possible because the grooves of the bobbin body 1-a prevent the drifting that disturbs the wire alignment as shown in FIG. 6. On the first coil layer of winding, drifting of the wire is inhibited by the grooves of the bobbin body 1-a, and on the second and after coil layers, the wound wire on the previous coil layer achieves the same function as grooves.
However, there are problems about the grooved bobbin, shown in FIG. 7, in that die manufacture cost is high. That is because antitype grooves must be created on a die, e.g., when the bobbin is made by a plastic molding. In addition, die alteration cost is generated when design conditions, such as a wire diameter, wire shape, shape of the bobbin, etc., are changed because the shape of grooves and a groove pitch are prescribed on the die.
On the other hand, in the method according to JP-A-2003-244906, because a wire is guided from outside of the bobbin, previously mentioned disturbance (the wire drifting) in wire alignment is prone to occur, causing a problem in accuracy.